FRIDAY, July 7 (HealthDay News) -- Having sex late in pregnancy does not help trigger labor and delivery, experts report.

In fact, a new study has found just the opposite: Women who were sexually active in the final three weeks of their pregnancies stayed pregnant slightly longer, delivering at an average of 39.9 weeks gestation vs. 39.3 weeks for women who did not have sex during that time.

"Sexual activity didn't have an effect," confirmed Dr. Jonathan Schaffir, author of a paper appearing in the June issue of Obstetrics and Gynecology and assistant professor of obstetrics and gynecology at Ohio State University in Columbus.

But for other experts, the findings were mainly seen as irrelevant.

"I don't think they answered the question. I'm not sure we need an answer to that question," said Dr. Victor Hugo Gonzalez-Quintero, director of Maternal Fetal Medicine at the University of Miami Miller School of Medicine. "What patients need to know is that sexual activity is acceptable in most of the typical patients, all the way to term."

Of all the old wives' tales involving pregnancy, this one may be the most appealing, Schaffir said.

"People look for all kinds of things to shorten the whole pregnancy process," he said.

One recent survey found that 73.6 percent of pregnant women knew about this theory, and that 46.1 percent believed it.

There is some biological plausibility to the notion, Schaffir said.

"Semen contains prostaglandin, which is a hormone that is in commercially prepared medications that we use to induce labor," he explained. "So, there is a thought that maybe sex is doing the same thing as commercial preparations. Also, maternal orgasm is associated with contractions," he added.

But to see if this notion had any basis in fact, Schaffir enrolled 93 women from his practice in a study.

All women had low-risk, single pregnancies and were at their 37th week of pregnancy or beyond at the time the study was conducted between July 2004 and July 2005.

"Over the course of routinely scheduled prenatal visits at the end of pregnancy, when they start coming every week, I asked them if they had had sex in the week prior and how many times," Schaffir said. "I was also doing cervical exams, which is generally a routine part of prenatal treatment."

Slightly more than half -- 50.5 percent -- of the women said they had had sex during the final weeks of their pregnancy. That's a higher percentage than was seen in prior studies, Schaffir noted.

However, the study found no correlation between frequency of sexual intercourse and cervical changes. There was also no appreciable difference in delivery dates between the two groups.

Women who had had sex delivered four days later, on average, than those who did not engage in intercourse, the study found. But that's not a significant difference, according to Schaffir.

The study is not the final word on the issue, the Ohio doctor said. But Schaffir added that it's difficult, if not impossible, to conduct a randomized trial on this subject.

The study did have one piece of reassuring news: The researchers found no differences in fetal outcomes between women who had sex close to delivery and women who did not.

"This suggests that, in a low-risk population, having sex at the end of pregnancy is safe and very commonly performed," Schaffir said.